Shallow Foundation Failure Surface

[NS4U]

I have heard a number of rules of thumb for estimating the depth to the bottom of a general shear failure surface. Anwhere for 2B to 4B for cohesionless soils, and around B for cohesive soils.

I can't seem to find any document that talks about this. Could somone point me in the right direction? Thanks.

 

[Ron]

Check any of the classic soils texts....Bowles, Terzaghi, Sowers, etc.

 

[NS4U]

i have checked bowles as well as other texts and i dont see this discussed in them

 

[Ron]

I will assume you are referencing a bearing capacity failure of a shallow foundation. Depending on the failure mode, the shear plane might be a bit hard to define. If it is just a general shear failure, then check Terzaghi's bearing capacity analysis approach.

If there is rotation of the foundation or an eccentric failure, it gets increasingly more difficult to define the failure surface. If it just dropped straight down with no side bulges, it is likely a punching shear failure.

 

[reyv]

2B to 4B is generally the depth of influence for settlement. If you want to be conservative, you can assume that the failure surface will be 2B for square footing and 4B for continuous footings for granular and fine-grained soils. Generally B is approximately where the failure surface will be for shallow foundations.

Rey Villa

http://geotech-apps.com

 

[Mccoy]

As far as bearing capacity is concerned, so when there is the presence of a defined failure surface, Mayerhof and Hanna 1978 is usually the cited source, with depth of influence depending upon friction angle and varying typically in the range 1B to 2B where B is width of the foudnation.

In bowles there is a relationship for the influence depth of the wedge, but it says nothing about the spirals. That's less than 1B and is in function of friction angle

These are the ones I know there might be others though

 

[joemacx]

Hi,

Have a look at book "Principles of Geotechnical Engineering" by Braja M. Das. There is a section near the end of the book which deals with this.

Joe.

 

[msquared48]

Lambe has some good guidelines for this too.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com